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What Is a Tundish, and Why Is Mine Dripping?
A tundish is a small funnel with a visible air gap, fitted in the safety discharge pipe from an unvented hot water cylinder and sometimes from a boiler's pressure relief valve. Its job is to let you see when a safety valve is releasing water, and to stop dirty water from the drain being drawn back into the system. A tundish should be dry. Water dripping into it means a safety valve is opening, most often because the cylinder's expansion vessel has lost its air charge or the pressure reducing valve is letting too much mains pressure through. Cold drips need fixing soon; hot water or steam pouring through it means the cylinder may be overheating, so switch off the boiler and immersion and call Emergency Plumber Chelsea on 020 3002 6667.
Updated July 2026 · Emergency Plumber Chelsea
What it looks like. On most unvented cylinders the tundish is a short piece of white plastic or chrome fitting, shaped like a small funnel, a little way below the cylinder's safety valves. A pipe comes down from the valves above, stops, and then there is an open gap with a slot or window where you can see into the funnel before the pipe continues down to the drain or an outside wall. That visible gap is the whole point. If you have an unvented cylinder in an airing cupboard, a kitchen cupboard or a utility room in a Chelsea flat, the tundish is usually within easy view of the cylinder.
Why it is there. An unvented cylinder holds hot water at mains pressure, so it has safety devices that release water if the pressure or temperature climbs too high. The water they release has to go somewhere safe, through a discharge pipe to a drain or to a point outside where it cannot scald anyone. The tundish sits in that pipe. It gives a visible warning that a valve is discharging, which would otherwise go unnoticed inside a wall, and the air gap means water from the drain side can never be siphoned back into the drinking water supply. Building regulations require it, and it must never be removed, capped or blocked.
Which valves feed into it. Most unvented cylinders have two. The expansion relief valve opens if the pressure in the cylinder rises too high, which usually happens as water heats and expands. The temperature and pressure relief valve, fitted near the top of the cylinder, opens if the water gets dangerously hot or the pressure climbs well past normal. On some installations the boiler's own pressure relief valve discharge pipe is also run into the same tundish, so a drip is not always coming from the cylinder. Our guide to a boiler pressure relief valve leaking covers that side.
The most common cause: the expansion vessel. When water heats up it expands, and on an unvented system that extra volume is taken up by an expansion vessel, a small tank, often red, white or blue, fitted near the cylinder or built into its casing. Inside there is a pocket of air that squashes as the water expands. Over time that air charge leaks away. Once it has gone, the expanding water has nowhere to go, the pressure rises every time the cylinder heats, and the expansion relief valve lets a little out into the tundish. The tell-tale pattern is a drip that starts while the water is heating and stops when it is up to temperature. Recharging the vessel, or replacing it if the internal membrane has failed, usually cures it.
The second common cause: the pressure reducing valve. Mains pressure in London varies, and it is often higher at night when demand drops. An unvented cylinder has a pressure reducing valve on the incoming cold supply to keep the pressure at a steady level. If that valve wears or fails, full mains pressure can creep into the cylinder and push the expansion relief valve open. A drip that happens even when the water is not heating, or mostly overnight, points this way. Our guide to the pressure reducing valve explains how it works and how it is tested.
Other causes of a dripping tundish. Grit or scale can lodge on the seat of a relief valve after it has opened once, so it never quite closes again and dribbles constantly. Valves also simply wear out with age, especially in London's hard water. And if someone has recently worked on the system, the pressures may not have been set up correctly afterwards. None of these is an emergency in the moment, but each one means the safety system is not working as it should.
When it is an emergency. If hot water, or steam, is pouring or spurting through the tundish, the temperature and pressure relief valve has probably opened because the water in the cylinder has overheated. That usually means both the cylinder thermostat and the high-limit cut-out on the boiler or immersion side have failed. Switch off the immersion heater at its switch and turn the boiler off. Do not touch the discharge pipe, which can be very hot, and keep children away from the outlet outside. Then call an engineer straight away. Our guide to a leaking hot water cylinder covers the wider steps.
What to do about a slow drip. First, check whether it is hot or cold and note when it happens: while the water is heating, all the time, or mainly overnight. That information helps an engineer find the cause quickly. Put a container under the discharge outlet if it ends somewhere it could cause damage, and keep an eye on the ceiling below if the pipe runs through a floor. Never tape over, cap, block or tie up the tundish or the discharge pipe to stop the drip. The drip is the symptom; blocking it removes the cylinder's safety release and turns a small repair into a dangerous one.
Who should fix it. Work on unvented cylinders should only be done by someone qualified for unvented hot water systems, often called G3 qualified after the part of the building regulations that covers them. Recharging an expansion vessel, replacing a relief valve cartridge or a pressure reducing valve are routine for the right engineer. Manufacturers generally recommend an annual service of an unvented cylinder, which includes checking the expansion vessel charge and testing the relief valves, and that is the best way to stop a tundish drip from starting in the first place.
Tundishes on vented systems. Older vented cylinders fed from a cold water tank in the loft do not have a tundish, because they are open to the atmosphere through a vent pipe and do not build up pressure. If you have a tundish, you have a pressurised system. Our guide to vented and unvented cylinders explains the difference.
Getting it checked. Emergency Plumber Chelsea diagnoses dripping tundishes, recharges and replaces expansion vessels, and replaces relief and pressure reducing valves across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We explain the cause and the options before any work starts, and there is no call-out fee on jobs over 3 hours. For hot water pouring from a tundish, call at any time. See our plumbing repairs and boiler repair pages, or call 020 3002 6667.
Quick tips
- A tundish should be dry. Any water in it means a safety valve is opening.
- Note whether the drip is hot or cold and when it happens; it points straight to the cause.
- Drips while the water heats usually mean the expansion vessel has lost its air charge.
- Hot water or steam through the tundish: switch off the immersion and boiler and call an engineer.
- Never cap, block or tape up a tundish or discharge pipe.
FAQs
Is a dripping tundish dangerous?
A slow cold drip is not an immediate danger, but it means a safety valve is opening and the fault behind it should be fixed soon. Hot water or steam coming through the tundish is dangerous: switch off the immersion heater and boiler and call an engineer straight away.
Why does my tundish drip when the hot water is heating?
That pattern usually means the expansion vessel has lost its air charge, so there is nowhere for the heated water to expand into and the expansion relief valve releases it. Recharging or replacing the vessel normally stops it.
Can I block off a dripping tundish?
No. The tundish and discharge pipe are part of the cylinder's safety system. Blocking them stops the safety valves releasing pressure and could lead to the cylinder failing. Fix the cause instead.
Where does the water from a tundish go?
Into a discharge pipe that runs to a drain, a gully or a safe point outside the building where hot water cannot hurt anyone. If you cannot tell where yours ends, an engineer can trace it during a service.
Does a combi boiler have a tundish?
A combi boiler has no hot water cylinder, so it has no cylinder safety valves feeding a tundish. Its pressure relief valve usually discharges through a pipe to the outside wall, although some installers run it into a tundish.
Need an engineer in Chelsea?
Call 020 3002 6667 for 24/7 emergency dispatch across SW3, SW10 and nearby postcodes.